American U.S. Pat. No. 6,707,367 discloses an on-chip multiple tap transformer balun having a first winding coupled to a second winding in different on-chip layers. The second winding has a first node, a second node and a center tap, and first and second portions between the center tap and first node and second node, respectively. The first and second portions of the second winding are symmetrical and arranged in one layer, to provide a similar coupling to the first winding.
American patent publication US2003/137383 discloses a transformer balun with a symmetrical structure, having primary and secondary metal windings at separate layers. The windings can be provided with a center tap, for example, for connection to ground, which allows a winding to be used as a differential port.
American patent publication US2005/052245 discloses an RF integrated circuit having an antenna diversity structure. Two antenna inputs are connected to corresponding (separate) transformer baluns and from there to two low noise amplifier inputs or two power amplifier outputs. For the transformer baluns used reference is made to American patent publication U.S. Pat. No. 6,882,263 having coupled windings in separate layers.